My comments about attacks requiring a target stems from two places. First, that the rulebook says you must choose a legal target during the declare attack step, and second from
>>>this post<<< in which Shad0w supports the idea of a target being required.
You do bring up an interesting point about Hellfire Trap, though. I can not find any official word on how the trap works with regards to needing to target the creature. It is not mentioned in the Rules Supplement, nor could I find it with a forum search. The only thing my search pulled up was a discussion you had previously regarding teleport trap not needing to target.
So, if the idea that attacks innately need a target is accurate, then we may end up with an issue where Hellfire Trap actually is not able to attack the Invisible Stalker, and is instead revealed without effect. That doesn't sound right to me, but I can't think of another way it would work without eliminating the need for attacks to have a target.
So, now I want an official word on both Gate to Hell and Hellfire Trap, lol. This question has gotten a lot more interesting than it first appeared.